1 stick (4 ounces) cold unsalted butter, cut into pieces
5 tablespoons ice water
2 cups milk
1 tablespoon cinnamon, plus more for dusting
4 large eggs
1 cup heavy cream
1 tablespoon confectioners' sugar
. In a medium saucepan, combine the tap water with 1/4 cup of the granulated sugar and bring to a boil. Off the heat, add the prunes and let stand until plumped, at least 4 hours or overnight. Drain the prunes, pressing out the liquid. Finely chop the prunes.
In a food processor, pulse the flour and salt with 1 tablespoon of the granulated sugar. Add the butter and pulse just until it is the size of peas. Lift the lid, sprinkle on the ice water and pulse just until the pastry is evenly moistened. Turn the pastry out onto a work surface and gather it into a ball. Flatten the pastry into a disk, wrap it in plastic and refrigerate for at least 30 minutes, or until firm.
Preheat the oven to 375°. On a lightly floured surface, roll out the pastry to a 13-inch round. Transfer the pastry to an 11-inch fluted tart pan with a removable bottom. Trim the overhang and refrigerate until chilled, about 20 minutes.
Line the pastry with foil and fill it with pie weights or dried beans. Bake in the lower third of the oven for about 40 minutes, until the crust is set. Remove the foil and weights and bake for about 15 minutes longer, just until the crust is golden and cooked through. Transfer to a rack to cool. Reduce the oven temperature to 325°.
In a small saucepan, combine the milk with the cinnamon and bring to a simmer, whisking constantly. In a medium bowl, whisk the eggs with the remaining 1/2 cup plus 2 tablespoons of granulated sugar. Gradually whisk in the warm cinnamon milk. Pour the custard into the crust and bake for 45 to 50 minutes, until just set. Transfer the tart pan to a rack to cool completely, then refrigerate for 2 hours, until completely chilled.
In a medium bowl, beat the heavy cream with the confectioners' sugar until firm peaks form. Spread the chopped prunes all over the top of the tart, then spoon the whipped cream on top. Dust lightly with cinnamon, cut the tart into wedges and serve.